1. Field of the Invention
This invention relates to a method and apparatus for treating a boil-off gas in low temperature liquid storage tanks. More particularly, it relates to a method and apparatus for treating a BOG generated in low temperature liquid tanks which are used for storing and transporting low temperature liquids obtained by liquefying various types of gases including methane, ethane, propane and other low hydrocarbons, natural gas, and carbon dioxide. It will be noted that the boil off gas may be sometimes referred to simply as BOG.
2. Prior Art
For storage and transportation of low temperature liquefied gases such as liquefied natural gas (LNG), it is usual to use storage tanks, and the storage tanks are thermally insulated with heat insulators. Nevertheless, heat is liable to enter into the inside of the tank from the external environments of the tank, and thus, part of the liquefied gas in the tank is gasified or evaporated as BOG, thereby causing the inner pressure to increase in the tank. The increase of the inner pressure in the tank may involve some risks such as of the leakage and explosion of gas. In order to avoid the above problem, it may occur that BOG generated in the tank is released therefrom. However, where the BOG consists of a flammable gas or a noxious gas, it cannot be released to the air.
As a measure for solving the above problem, it has been considered to transfer the BOG generated in a low temperature liquid storage tank to a separate vessel. For the transfer, (1) the BOG is transferred by use of a natural stream thereof, i.e. the transfer is carried out by utilizing a stream through a pressure gradient based on the difference in pressure between the low temperature liquid storage tank and the vessel. In this case, however, the pressure in the vessel should be lower than that in the storage tank, thus requiring a pressure reducing device.
(2) Where the transfer cannot rely on a natural stream, a suction pump for forcedly transferring the BOG is necessary. In this case, the transfer pump for that BOG should be provided, for example, at a pipe between the storage tank and the vessel. This promotes gasification and evaporation in the tank the would not occur otherwise. Since the tank and a pipe communicating therefrom are kept at low or very low temperatures, an ordinary type of suction pump cannot be provided, with the attendant problem that a specific type of pump is necessary.
Besides, although it does not differ from the above in that the BOG generated in the low temperature liquid storage tank is transferred via a pipe to a separate vessel other than the storage tank, (3) a vessel used may be one wherein an adsorbent is packed, and the generated BOG is adsorbed to the adsorbent. In fact, this is proposed in JP-A- 8-219397. In this case, the gas is subjected to physical adsorption on the surfaces of a solid adsorbent. The physical adsorption makes use of a phenomenon of equilibrium with pressure, so that not only the adsorption rate is low, but also the adsorption amount is small, with a large amount of adsorbent being required. Therefore, in order to enable the generated BOG to be removed and adsorbed satisfactorily, a vessel with a great capacity and packed with a large amount of an adsorbent becomes necessary.
We have developed a method of storing and transporting gas wherein a large amount of gas can be stored and transported by bringing a compound serving as host and a gas to be stored into contact with a porous material under mild temperature and pressure conditions in a very short time, thereby enabling the gas in an amount, for example, equivalent to not less than 180 times (converted to the standard state basis) as much as an unit volume of the porous material to be stored or transported (Japanese Patent Application No. 8-37526). The porous materials used in this method include active carbon, ceramics and the like, and the host compounds include water, alcohols, organic acids, quinones, hydrogen sulfide, urea and the like.
FIG. 1 is a graph showing an example evidencing the characteristics of the porous material. In this instance, after 0.0083 g of water was adsorbed to 0.0320 g (0.0461 cc) of pitch-based active carbon having a specific surface area of 1765 m.sup.2 /g, an average pore size of 1.13 nm (nanometers), a pore capacity of 0.971 cc/g, an intrinsic specific gravity of 2.13 g/cc, and an apparent specific gravity of 0.694 g/cc, methane gas under 0.2 atm at 30.degree. C. was fed thereto. For comparison, the above procedure was repeated except that methane gas under the same conditions as mentioned above was fed thereto but without adsorption of any water. In FIG. 1, the variation in the weight of the methane gas adsorbed per 1 g of the active carbon is shown in relation to the variation in time. In FIG. 1, the variation in the gas weight when water was adsorbed to the active carbon prior to the methane gas being adsorbed thereto is plotted with the mark ".smallcircle." (blank circles), whereas the variation obtained when methane gas was adsorbed straight to the active carbon is plotted with the mark ".circle-solid." (solid circles).
As shown in FIG. 1, where water was adsorbed to the active carbon first and then the methane gas was fed thereto, the active carbon started to adsorb the methane gas henceforth at a rapid rate with the amount of the methane gas adsorbed after the elapse of 0.2 hours reaching more than 15 mmols per 1 g of the active carbon and the same after the elapse of 0.5 hours reaching around 17 mmols per 1 g of the active carbon, which was maintained thereafter. Considering the fact that the methane gas fed at this point was pressurized at 0.2 atm (at 30.degree. C.), it can be seen that the rate at which the methane gas is adsorbed is remarkable.
On the other hand, when the methane gas was fed without water being adsorbed to the active carbon beforehand as in the conventional methods, only a minimal amount of the methane gas was adsorbed without showing any change in the amount of the methane gas adsorbed after the elapse of time under the same pressure as described above. In this regard, according to the method referred to in JP-A- 49-104213, for example, silica gel, molecular sieves, active carbon, and the like are placed in a pressure tank, and methane gas is stored by applying pressure at about 68 atm (equivalent to 1000 psia). In application of this technique, such a high-pressure operation is indispensable even using similar adsorbents described above.
Table 1 shows the results of the comparison of the amounts of methane adsorbed per 1 g of the active carbon as shown in FIG. 1. As shown in Table 1, the amount of methane adsorbed was only 0.18 mmols after the elapse of 0.2 hours in the case where methane was adsorbed straight to the active carbon, whereas it was 12.08 mmols in the case where water was adsorbed to the active carbon beforehand, 67 times as much as the former case. After the elapse of 0.9 hours, the amount of methane adsorbed straight to the active carbon was 0.18 mmols, whereas it was 16.4 mmols in the case of water coexisting with methane, 91 times as much as the former case.
TABLE 1 ______________________________________ Amount of methane adsorbed per 1 g of active carbon (mmols) Methane adsorbed Methane Time to the active carbon adsorbed Elapsed after water was straight to the Ratio (Hours) adsorbed (A) active carbon (B) (A/B) ______________________________________ 0.2 12.08 0.18 67.1 0.9 16.46 0.18 91.4 ______________________________________
The volume of methane adsorbed to 1 cc in an apparent volume of the active carbon in the presence of water is calculated as 183 cc on the standard state basis under 1 atm at 0.degree. C. This result shows that methane was stored in a volume exactly 183 times, on the standard state basis, as large as an unit volume of the active carbon under a pressure as low as only 0.2 atm. Then (after the elapse of 0.9 hours), the amount of methane adsorbed was found slightly reduce, and finally reached 11.77 mmols, at which a state of equilibrium was achieved without any change thereafter.